Remove @slow decorators for tests that are no longer slow.

These tests aren't exactly zippy, but they don't even make
the top 100 slow tests.

zerver/tests/test_hooks.py

@@ -235,7 +235,6 @@ class BeanstalkHookTests(WebhookTestCase):
                                           content_type=None,
                                           **self.api_auth(self.TEST_USER_EMAIL))
 
-    @slow(0.20, "lots of queries")
     def test_git_multiple(self):
         # type: () -> None
         expected_subject = "work-test"

zerver/tests/test_signup.py

@@ -450,7 +450,6 @@ so we didn't send them an invitation. We did send invitations to everyone else!"
             self.invite(external_address, ["Denmark"]),
             "Some emails did not validate, so we didn't send any invitations.")
 
-    @slow(0.20, 'inviting is slow')
     def test_invite_outside_domain_in_open_realm(self):
         # type: () -> None
         """